While polymer blends have been reported in the literature from time to time, only within the past generation have polymer blends become sufficiently numerous and the advantages of polymer blends sufficiently apparent to warrant widespread investigation. While originally considered rare, miscible polymer blends more recently have been noted with increasing frequency. Miscible blends of a second polymer and a first polymer have found utility in providing enhanced properties such as plasticization, tensile strength, melt processability, and increased resistance to heat distortion. Particularly in applications where polymeric materials are to be subjected to injection molding conditions, it is desirable that the modifying polymers blended into a polymer being injection molded be miscible in the first polymer; miscibility enhances the opportunity for avoiding weld-line strength difficulties and defects in finished, injection molded parts.
The prediction of miscibility between polymer pairs is still an art in infancy; miscibility is believed dependent upon a number of factors that include reactions between functional moieties pendant from one or more of the polymers, hydrogen bonding, and the like. Various suggestions have appeared for assisting in the selection of miscible polymer pairs including an application of Flory's equation of state as set forth by L. P. McMaster, 6 Macromolecules, 760 (1973).
Other suggestions for useful tools in assessing miscible polymer pairs have included: two-dimensional solubility parameters, inverse gas chromatography; crystallization characteristics of polymer blends; and evaluation of glass transition temperature shifts, as suggested by L. M. Robeson, 24 Polymer Engineering and Science, p.p. 589 (June 1984). That the prediction of miscible polymer pairs is still an art, rather than a science, is indicated by, for example, by chlorinated polyethylene having 42 weight percent chlorine being miscible in polyvinyl chloride, while chlorinated polyethylene having a chlorine content less than 42% being immiscible in polyvinyl chloride as shown by Robeson, supra. at p.p. 588.
The prediction of properties of miscible blends is also uncertain. While some blend properties such as tensile strength may follow simple additivity rules, for example as noted for polycarbonate/polyester blends by Joseph, E. A., et al 23 Polymer p.p. 112 (1982), other blends may show synergistic enhancement of tensile properties as discussed by Olabisi, O., et al, 176 Adv. Chem. Serv., p.p. 559 (1979). Blends characterized by the components being immiscible typically are found to exhibit a tensile strength minimum while blends having components characterized by slight miscibility and a dual glass transition temperature may exhibit both a tensile minimum and a maximum as discussed by Fried, J. R. et al, 50 Journal of Applied Physics, p.p. 6052 (1979).
There is some significant indication that polymer blends tend to exhibit partial miscibility, and that graphical depictions of such partial miscibility tend to be of the minimum solution temperature type, that is those solubility curves having a minimum critical solution temperature below which a polymer pair exists in miscible state and above which, two phases are present, one phase being rich in a first polymer and the second phase being rich in a second polymer; Robeson, supra. p.p. 588. One possible explanation for a lack of solubility between polymer pairs may, in some cases, be related to the minimum critical solution temperature being lower than a glass transition temperature for one or both of the polymers; the polymers being below a glass transition temperature; a melt state for one or both of the polymers being non existent.
A number of substances forming a miscible melt with polyvinyl chloride have been identified in the literature, Robeson, supra. at p.p. 588, however, chlorinated polyvinyl chloride appears to have received less attention. Polyvinyl chloride and chlorinated polyvinyl chloride being chemically different compounds and particularly where properties of a miscible blend of two polymers is in part dependent upon such factors as hydrogen bonding and pendant functional moieties, the simple fact that polyvinyl chloride forms a miscible pair with a second polymer is not a particularly good indicator that chlorinated polyvinyl chloride will also form a miscible pair with the second polymer.
While chlorinated polyvinyl chloride is possessed generally of performance characteristics and, particularly temperature performance characteristics, superior to polyvinyl chloride, enhanced performance characteristics associated with a blend of chlorinated polyvinyl chloride and a second polymer together in a miscible melt state could find substantial industrial utility.